IPT technology is an area of increasing development and IPT systems are now utilised in a range of applications and with various configurations. One such application is the use of IPT systems in so called ‘charging mats’. Such charging mats will normally provide a planar charging surface onto which portable electronic devices (such as smartphones or PDAs) may be placed to be charged or powered wirelessly.
Typically, the charging mat will include a transmitter having one or more transmitting coils arranged parallel to the planar charging surface of the charging mat. The transmitter drives the transmitting coils so that the transmitting coils generate a time-varying magnetic field in the immediate vicinity of the planar surface. When portable electronic devices are placed on or near the near the planar surface, the time-varying magnetic field will induce an alternating current in the receiving coil of a suitable receiver associated with the device (for example a receiver incorporated into the device itself). The received power may then be used to charge a battery, or power the device or some other load.
A problem associated with charging mat design is ensuring that the inductive power transfer is adequately efficient. One approach is to require precise alignment between the transmitting coil and the receiving coil. This may be achieved, for example, by having markings or indentations on the planar charging surface so that when a user places the device on the charging mat alignment between the coils can be guaranteed. However, this approach is not ideal since it requires the user to place their device carefully onto the charging mat. Further, if a charging mat is to be used by multiple devices, then there needs to be a suitable number of transmitting coils arranged in the charging mat.
Another approach is to have a single large transmitting coil which corresponds to the entire surface of the charging mat. In this instance, one or more devices may be placed anywhere on the surface of the charging mat. This allows more freedom in terms of where a user may place a device on the charging mat. However, the magnetic field produced by a large transmitting coil may not be uniform, with ‘weak spots’ towards the centre of the charging mat.
Another approach is to have an array of smaller transmitting coils. The charging mat detects the position of the device using a suitable detection mechanism and activates the most proximate transmitting coil or coils. Though this allows more freedom in terms of where a user may place a device, it requires that each transmitting coil can be driven individually and separate detection circuitry which makes the charging mat more complex and expensive. Also, the boundary between adjacent transmitting coils can result in deadspots due to the cancelling effects of adjacent coils whereby receivers do not receiver sufficient power.
Another aspect of charging mat design is consideration of the relative orientation of the transmitting coils and receiving coils. For the approaches discussed above, the receiving coil will typically need to be parallel with the transmitting coil. This is usually achieved by positioning the receiving coil in the portable electronic device such that when the device is placed on the charging mat in the normal way the receiving coil is parallel to the surface of the charging mat and the transmitting coils. However, in some scenarios when a device is placed on the charging mat the receiving coil may not be parallel to the planar surface. This may be due to the device been placed incorrectly, the receiver been adapted for another type of transmitter (such as a charging enclosure), or for any other reason. Mis-orientation may result in less efficient power transfer.
The present invention provides a transmitter for a charging mat that resolves at least some of the problems identified above. In particular, a charging mat that produces a magnetic field which can transfer power to multiple devices placed onto the charging mat regardless of their orientation, with minimal deadspots, or at least provides the public with a useful choice.